The majority of known sludge collectors of the above type possess moving parts, which are located on, in the vicinity of, the bottom of the basin.
So-called blade-type collectors possess a clearing-blade, which is periodically moved along the bottom and pushes the sludge which has settled into a trough formed in the bottom of the basin, whence the sludge is drawn off from the trough by means of the suction pump. As a rule circular settling basins are provided with a so-called rotary collector, whose clearing-blade starts from the centre of the basin and executes a rotary movement so that the sludge which has settled gradually moves into a depression which is located at the periphery or in the middle of the bottom of the basin.
So-called chain-type collectors function in a similar manner, in which clearing-beams are located at intervals, between two endless, parallel revolving chains, these beams sweeping the bottom of the basin in the manner of a dredger and conveying the sludge which has settled into a trough formed in the bottom.
Another known sludge collector is the so-called suction-type collector, in which the bottom of the basin is swept by a pipe having inlet-openings or inlet-slots and which is connected to a suction pump, in a manner similar to the suction-nozzle of a vacuum cleaner.
All these sludge collectors, which are provided with parts which are themselves moved in the basin, give rise to flows in the contents of the basin, which at least disturb the settling of the sludge. Moreover, these known collectors are subject to mechanical faults which necessitate expensive maintenance operations.
In this regard, so-called funnel-basins constitute an exception; in these the bottom of the basin has the form of a conical or pyramidal funnel, or of a succession of several such funnels superimposed on each other, with a suction line emerging from the lowest point of this funnel, via which line the sludge is removed from the basin. However, apart from considerable construction costs, these funnel-basins have the disadvantage that the only sludge removed is that which actually reaches the lowest point of the funnel, whilst other sludge particles deposit on the wall of the funnel and can there lead to accumulations of sludge, which are beyond the "range" of the suction line.